1. Field of the Invention
This invention relates to the field of cables and cable terminations. More specifically, the invention comprises a cable termination which redistributes stress in order to enhance the mechanical properties of the termination.
2. Description of the Related Art
Devices for mounting a termination on the end of a wire, rope, or cable are disclosed in detail in U.S. application Ser. No. 60/404,973 to Campbell, which is incorporated herein by reference. The individual components of a wire rope are generally referred to as “strands,” whereas the individual components of natural-fiber cables or synthetic cables are generally referred to as “fibers.” For purposes of this application, the term “strands” will be used generically to refer to both.
In order to carry a tensile load an appropriate connective device must be added to a cable. A connective device is typically added to an end of the cable, but may also be added at some intermediate point between the two ends. FIG. 1 shows a connective device which is well known in the art. FIG. 2 shows the same assembly sectioned in half to show its internal details. Anchor 18 includes tapered cavity 28 running through its mid portion. In order to affix anchor 18 to cable 10, the strands proximate the end of cable 10 are exposed and placed within tapered cavity 28 (They may also be splayed or fanned to conform to the expanding shape of the tapered cavity).
Liquid potting compound is added to the region of strands lying within the anchor (either before or after the strands are placed within the anchor). This liquid potting compound solidifies while the strands are within the anchor to form potted region 16 as shown in FIG. 2. Most of potted region 16 consists of a composite structure of strands and solidified potting compound. Potting transition 20 is the boundary between the length of strands which is locked within the solidified potting compound and the freely-flexing length within the rest of the cable.
The unified assembly shown in FIGS. 1 and 2 is referred to as a “termination” (designated as “14” in the view). The mechanical fitting itself is referred to as an “anchor” (designated as “18” in the view). Thus, an anchor is affixed to a cable to form a termination. These terms will be used consistently throughout this disclosure.
Cables such as the one shown in FIG. 2 are used to carry tensile loads. When a tensile load is placed on the cable, this load must be transmitted to the anchor, and then from the anchor to whatever component the cable attaches to (typically through a thread, flange, or other fastening feature found on the anchor). As an example, if the cable is used in a winch, the anchor might include a large hook.
Those skilled in the art will realize that potted region 16 is locked within anchor 18 by the shape of tapered cavity 28. FIG. 3 is a sectional view showing the potted region removed from the anchor. As shown in FIG. 3, tapered cavity 28 molds the shape of potted region 16 so that a mechanical interference is created between the two conical surfaces. When the potted region first solidifies, a surface bond is often created between the potted region and the wall of the tapered cavity. When the cable is first loaded, the potted region is pulled downward (with respect to the orientation shown in the view) within the tapered cavity. This action is often referred to as “seating” the potted region. The surface bond typically fractures. Potted region 16 is then retained within tapered cavity 28 solely by the mechanical interference of the mating male and female conical surfaces.
FIG. 4 shows the assembly of FIG. 3 in an elevation view. As mentioned previously, the seating process places considerable shearing stress on the surface bond, which often breaks. Further downward movement is arrested by the compressive forces exerted on the potted region by the tapered cavity (Spatial terms such as “downward”, “upper”, and “mid” are used throughout this disclosure. These terms are to be understood with respect to the orientations shown in the views. The assemblies shown can be used in any orientation. Thus, if a cable assembly is used in an inverted position, what was described as the “upper region” herein may be the lowest portion of the assembly). The compressive stress on potted region 16 tends to be maximized in neck region 22. Flexural stresses tend to be maximized in this region as well, since it is the transition between the freely flexing and rigidly locked regions of the strands.
The tensile stresses within potted region 16 likewise tend to be maximized in neck region 22, since it represents the minimum cross-sectional area. Thus, it is typical for terminations such as shown in FIGS. 1-4 to fail within neck region 22.
In FIG. 4, potted region 16 is divided generally into neck region 22, mid region 24, and distal region 26. Potting transition 20 denotes the interface between the relatively rigid potted region 16 and the relatively freely flexing flexible region 30. Stress is generally highest in neck region 22, lower in mid region 24, and lowest in distal region 26. A simple stress analysis explains this phenomenon.
Considering the stress placed on a thin transverse “slice” within the potted region is helpful. FIG. 5 shows thin section 60 within potted region 16. The potted region is held within a corresponding tapered cavity in an anchor. Seating force 62 pulls the potted region to the right in the view, thereby compressing the potted region.
FIG. 6 graphically illustrates the seating phenomenon. A coordinate system is established for reference. The X Axis runs along the center axis of anchor 18. Its point of origin lies on the anchor's distal extreme (distal to the neck region). The Y Axis is perpendicular to the X Axis. Its point of origin is the same as for the X Axis.
The thin section starts at unseated position 64. However, once seating force 62 is applied, the thin section moves to the right to seated position 66. The section moves through a distance ΔX. Significantly, the thin section is transversely compressed a distance ΔY. It must be compressed since the wall of tapered cavity 28 slopes inward as the thin section moves toward the neck region. The reader should note that the seating movement is exaggerated in the view for visual clarity.
FIG. 7 shows a plan view of the thin section. Unseated position 64 is shown in dashed lines. Seated position 66 is shown as solid. The thin section actually has a conical side wall (matching the slope of tapered cavity 28 within anchor 18). However, for a thin section this side wall can be approximated as a perpendicular wall without the introduction of significant error. With this assumption, the thin section becomes a very short cylinder, having a volume of π·r2h, with h being the thickness of the section (or, in other words, the height of the very short cylinder).
In FIG. 7, Y1 is the radius of the thin section in unseated position 64, while Y2 is the radius of the thin section in seated position 66. The volume of the section in the unseated position is π·Y12·h, while the volume of the seated position is π·Y22·h. A simple expression for compressive strain based on volume reduction is as follows:
  ɛ  =                              (                                    π              ·                              Y                1                2                                      -                          π              ·                              Y                2                2                                              )                ·        h                    π        ·                  Y          1          2                ·        h              =          1      -                        Y          2          2                          Y          1          2                    
The hoop stress occurring within the thin section is linearly proportional to the compressive hoop strain (or very nearly so). Thus, the hoop stress in the thin section can be expressed as:
            σ      hoop        ≈                  k        1            ·              (                  1          -                                    Y              2              2                                      Y              1              2                                      )              ,where k1 is a scalar.
Consider now the situation depicted graphically in FIGS. 8 and 9. The same anchor is used. The same tapered cavity having a straight side wall is used. In this analysis, however, two separate thin sections will be considered. Distal thin section 68 lies distal to the potting transition in the region of the neck. Neck thin section 70 lies within the neck region proximate the potting transition. If tension is placed on the cable while the anchor is held in place, the potted region will “seat” by shifting to the right in the view. Each of the thin sections therefore has an unseated position 64 (shown in dashed lines) and a seated position 66 (shown in solid lines).
FIG. 9 shows a plan view of distal thin section 68 and neck thin section 70. Again, the unseated position for both is shown in dashed lines while the seated positions are shown in solid lines. Because of the straight side wall within the anchor, the radius of both thin sections is reduced an amount ΔY. However, the reduction in area of the two sections will not be the same. The reader will recall from the prior expression that the hoop strain may be expressed as:
  ɛ  =      1    -                  Y        2        2                    Y        1        2            
Applying this equation to the two sections shown in FIG. 9, one may easily see that the smaller section (neck thin section 70) undergoes a greater strain than does the larger section (distal thin section 68), for a given amount of seating. A quick analysis using actual numbers makes this point more clear. Assume that the radius of neck thin section 70 in the unseated position is 0.250, while the unseated radius of distal thin section 68 is 0.350. Further assume that the seating movement produces a ΔY of 0.020. The strain for neck thin section 70 would be
      1    -          .                        .23          2                          .25          2                      =      .1536    .  The strain for distal thin section 68 would be
      1    -                  .33        2                    .35        2              =      .1110    .  
Thus, for a given amount of seating, the hoop strain increases when proceeding from the distal region to the neck region. Since the hoop stress is approximately linearly proportional to the hoop strain, the hoop stress is likewise increasing. FIG. 10 shows a representative plot of hoop stress plotted against position along the X Axis (the centerline of the cavity within the anchor). The reader will observe the stress concentration in the neck region. This stress concentration is undesirable, and represents a limitation of the prior art design. Thus, a goal of the present invention is to redistribute stress from the neck region to the mid and distal regions.